This invention relates to a method of predicting and processing image fine structures. More particularly, the invention relates to a method of predicting and processing image fine structures which, when applied to a system for preparing color proofs with an image output device such as a color printer or a CRT display before a printed color document carrying halftone dot images (or simply "halftone images") is actually produced with a color printing machine using rotary presses or the like that have press plates mounted thereon, can ensure that image fine structures such as moire and a rosette image which would occur in the actually produced printed document are represented in advance either as an image on the display such as CRT (the image is generally referred to as "soft proof") or as an image on a hard copy output from the color printer (which is generally referred to as "hard proof").
The process of producing printed documents carrying halftone images with a color printing machine using rotary presses or the like is not only time-consuming but also costly and, hence, it is common practice to produce proofs with a device other than the color printing machine which is commonly referred to as a proofer before the printed document is output as the actual product. The production of proofs with a proofer has two purposes, one for predicting the colors to be reproduced on the printed document (this may be called "simulation of color reproductions") and the other is for predicting the image fine structures to be produced on the printed document (which may be called "simulation of image fine structures").
Two types of proofers have heretofore been proposed for use in the production of proofs for printed documents, one being a proofer which involves dot formation and the other being a non-dot forming proofer. Proofing technology, or the techniques for predicting and processing image fine structures using proofers, is conventionally implemented by the following three methods. In the first method, a high-resolution digital printer is used as a dot-forming proofer and halftone images are actually produced. This technique has the advantage of correctly simulating the image fine structures which will appear on the printed document.
A dot-forming proofer is also used in the second method and halftone images (pictures) formed on printing lith films for four plates of C (cyan), M (magenta), Y (yellow) and K (black) are individually transferred to special chemical materials by exposure and the resulting four sheets of chemical materials for the C, M, Y and K plates which are commonly referred to as "transfer films" are placed one on another. An example of such proofers is one that employs the transfer films marketed by the Applicant. In this method, transfer films for the four plates of C (cyan), M (magenta), Y (yellow) and K (black) are prepared by dot formation using the actual screen ruling and transferred onto a print sheet at the actual screen angles to thereby produce a hard proof. This approach also has the advantage of correctly simulating the image fine structures which will appear on the printed document.
The third method uses a non-dot forming proofer which is exemplified by a system employing a continuous-tone color printer such as one which uses sublimable dye pigmented inks and which operates on a density modulation process to achieve a resolution of 300 dpi. This type of color printer represents the original image as a continuous-tone image without forming dots and, hence, has the advantage of producing proofs by simple procedures.
Of the two methods that use a dot forming proofer, the first approach which actually produces halftone dots with a high-resolution digital printer allows for halftone representation and is capable of correctly simulating the image fine structures which will appear on the printed documents. On the other hand, the high-resolution digital printer is an expensive device and requires high running cost; hence, the first method is not a convenient way to be performed in practice.
The second approach which superposes four transfer films for the plates of CMYK which are made of special chemical materials also allows for halftone representation and is capable of correctly simulating the image fine structures which will appear on the printed document. However, the apparatus used in the method is also costly. In addition, the cost of the print sheet is relatively high and the transfer films made of special chemical materials are also expensive. What is more, the process up to the stage of proof production is cumbersome (i.e., requires much labor due to manual operations) and, hence, a comparatively long time is taken to produce the desired proof; in other words, the second method is not necessarily an easy way to operate. In addition, it has been theoretically difficult to achieve color matching with the ultimate printed document.
In the third method which uses a non-dot forming proofer, the original image is represented as a continuous-tone image by means of a continuous-tone printer without producing dots. Therefore, this method is inexpensive, is convenient and can produce a proof within a short time. On the other hand, the method gives no consideration to representation by dots and is not capable of representing halftones; hence, the proof produced by the method can be used in simulating color reproduction but not in simulating image fine structures.
Under the circumstances, there has been a strong need for a technology that retains the advantage of low cost and convenience of the third method using a non-dot forming proofer and which yet is capable of representing dot-generated image fine structures as in the first and second methods which employ a dot-forming proofer.
Further referring to the third approach which uses a non-dot forming proofer, it has heretofore been customary to produce color proofs for examining and correcting colors and so forth before a printed color document carrying halftone images is ultimately produced by a color printing machine. The proofs are produced using a color printer that forms an image for each pixel by a density gradation process (also called "continuous gradation process") and this is primarily because the color printer is of a comparatively simple Composition and inexpensive. In addition, by means of the color printer, a hard copy having an image formed on a sheet can easily be produced a plurality of times within a short period of time since, as is well known in the art, the preparation of process-plate films and press plates which are required by color printing machines are not needed by the color printer.
FIG. 23 shows the flow of a conventional process for producing color proofs using a color printer. First, the image on an image document 52 is read two-dimensionally with an image reader such as a color scanner having a CCD linear image sensor or the like and gradation (continuous-tone) image data Ia for each of the colors R (red), G (green) and B (blue) are generated (step S51: image reading step).
Then, in step S52, the RGB gradation image data Ia are rendered by a color conversion process using conversion lookup tables or the like into dot area percentage data (also referred to as "dot percentage data" or "original image pixel dot percentage data") aj for the four plates of respective colors C (cyan), M (magenta), Y (yellow) and K (black), where j=0, 1, 2, 3 (0 represents the color C, 1 the color M, 2 the color Y, and 3 the color K). The color conversion process allows for various versions in relation to the color printing machine to be described later on and it is usually the proprietary know-how of individual printing companies who employ different color printing machines.
Halftone images are produced by the color printing machine and, hence, in order to produce a printed color document in the actual practice, the dot area percentage data aj produced by the color conversion process are rendered into bit map data, on the basis of which a process-plate film or the like is generated. A problem with the color printing machine is the need to use an automatic image developing machine, so the process following the generation of the process-plate film is considerably complicated.
To facilitate the production of color proofs, a color printer 53 (which may hereinafter be referred to as either "color digital printer" or "DP" as the case may be) is employed for the reasons set forth above. DP 53 forms an image on a donor film by a density gradation process in which the intensity and time of emission of three primary colors from an LED (light-emitting diode) or a laser are digitally controlled pixel for pixel and the image is transferred to an image-receiving sheet, whereby image formation is effected on the sheet. Compared to the color printing machine which generates presensitized plates from printing plates and which produces a printed color document using the presensitized plates, DP 53 is considerably inexpensive. In addition, it is smaller in volume and lighter in weight.
In order to employ DP 53, it is necessary that the halftone-dot area percentage data aj of the four CMYK plates produced in step S52 be converted into image data (also called "common color space data") which are independent of devices including a printing device, a CRT, a photographic device, an LED, etc. and which are exemplified by tristimulus value data X, Y, Z. To meet this need, the halftone-dot area percentage data aj of the four CMYK plates are converted into tristimulus value data X, Y, Z in an image data processing section (step S54). The image data processing is conventionally carried out using the Neugebauer's equation.
Prior to step S54, colorimetric data Xi, Yi, Zi (i represents 2.sup.4 =16 colors for the four CMYK plates and ranges from 0 to 15) for the colors of printing inks are measured with a calorimeter (step S53). To measure the calorimetric data Xi, Yi, Zi, the 16 colors are printed on a print sheet which will be used to produce a printed color document with a color printing machine, thereby preparing "color patches". This process is commonly referred to as "solid printing". The 16 colors correspond to the presence and absence of the respective colors, C, M, Y, K (2.sup.4 =16).
Specifically, the 16 colors consist of color w (white) which represents the background color of the print sheet when nothing is printed on it, the primary colors C, M, Y, color K (black), and mixed colors C+M, C+Y, C+K, M+Y, M+K, Y+K, C+M+Y, C+M+K, C+Y+K, M+Y+K, and C+M+Y+K. These 16 colors are also called "16 basic colors". The colors of reflection from the colors printed on the print sheet are measured with a colorimeter such as a spectrometer to produce the calorimetric data Xi, Yi, Zi.
In the image data processing using the Neugebauer's equation, the colorimetric data Xi, Yi, Zi are multiplied by the area percentage data bi (i=0-15) as a coefficient [(see the following equations (6)] to produce the tristimulus value data X, Y, Z which have been subjected to image data processing (step S54): EQU X=.SIGMA..sub.i=0.sup.15 bi.multidot.Xi EQU y=.SIGMA..sub.i=0.sub.15 bi.multidot.Yi EQU Z=.SIGMA..sub.i=0.sup.15 bi.multidot.Zi (6)
The area percentage data bi of the 16 basic colors which are included as a coefficient in equations (6) are determined from the halftone-dot area percentage data aj by performing probability calculations as follows: EQU b0=(1-c) (1-m) (1-y) (1-k) EQU b1=c.multidot.(1-m) (1-y) (1-k) EQU b2=(1-c).multidot.m.multidot.(1-y) (1-k) EQU b3=c.multidot.m.multidot.(1-y) (1-k) EQU b4=(1-c) (1-m).multidot.y.multidot.(1-k) EQU b5=c.multidot.(1-m).multidot.y.multidot.(1-k) EQU b6=(1-c).multidot.m.multidot.y.multidot.(1-k) EQU b7=c.multidot.m.multidot.y.multidot.(1-k) EQU b8=(1-c) (1-m) (1-y).multidot.k EQU b9=c.multidot.(1-m) (1-y).multidot.k EQU b10=(1-c).multidot.m.multidot.(1-y).multidot.k
b11=c.multidot.m.multidot.(1-y).multidot.k EQU b12=(1-c) (1-m).multidot.y.multidot.k EQU b13=c.multidot.(1-m).multidot.y.multidot.k EQU b14=(1-c).multidot.m.multidot.y.multidot.k EQU b15=c.multidot.m.multidot.y.multidot.k (7)
To provide for easy understanding by intuition, the halftone-dot area percentage data aj (j=0-3) are set to a0=c, a1=m, a2=y and a3=k in the above equations (7) and c, m, y and k represent the halftone-dot area percentage data of the respective color plates C, M, Y and K. Take, for example, b3 which represents the area percentage of the mixed color C+M in the equations (7); this parameter can be determined by multiplying c (the probability that plate C exists), m (the probability that plate M exists), 1-y (the probability that plate Y does not exist), and 1-k (the probability that plate K does not exist). Therefore, the Neugebauer's equation expressed by the equations (7) can be understood as being based on the theory of probability.
The tristimulus value data X, Y, Z thus obtained by image data processing according to equations (6) are supplied to DP 53, in which they are converted into data for the three primaries with respect to the laser beam or the like on the basis of lookup tables (LUTs). Said data are so-called "device dependent image data", which are sometimes referred to as "inherent color space data". Thereafter, DP 53 generates a color proof CPa which is a hard copy having an image formed on a sheet (step S54).
When the tristimulus value data X, Y, Z for DP 53 are generated using the Neugebauer's equation as described above, the colors of the printed color document to be produced with a color printing machine can be reproduced faithfully in the image on the hard copy due to the use of the calorimetric data obtained by measurement with a calorimeter as representing the colors of the image to be formed on the printed color document. On the other hand, image fine structures which will appear on the printed color document, such as moire, a rosette image and other peculiar patterns caused by interference fringes cannot be reproduced in the image on the hard copy.
If image fine structures are to appear on the printed color document, they should also be reproduced faithfully on the color proof CPa which is output from DP 53. In this respect, the conventional color proof CPa which fails to reproduce image fine structures is not an accurate (faithful) proof for the printed color document.
The reason for the failure of image fine structures to appear on the hard copy from DP 53 is conceivably because the Neugebauer's equation is based on the theory of probability as described above.
Under the circumstances, the present inventors made intensive studies in order to verify the hypothesis that if pixel data which compose input image data for use with a color printer are generated without relying upon the Neugebauer's equation, image fine structures such as moire, a rosette image and so forth which are peculiar to the printed document to be produced can be reproduced on a color proof in an accurate and faithful manner. As a result, the inventors proposed in Unexamined Published Japanese Patent Application (kokai) No. Hei 8-192540 a technique which is capable of faithful reproduction of not only the colors of a printed image but also the image fine structures such as moire and a rosette image which appear due to halftoning.
This technique enables the simulation of interference-generated image fine structures using a continuous-tone printer but at the same time it suffers from the problem of taking time in processing. The reason for the slow processing speed of this technique is the great number of mathematical operations to be performed on individual pixels, which in turn is caused by the need to simulate halftone dots by performing the same calculations for each pixel as are effected in the halftoning step in the printing process.